1. Field of the Invention
The invention relates to a protective circuit for electrical connection to solar cells of a solar cell module, with a protective means which, in the case of a shaded solar cell, acts as a current bypass for the shaded solar cell.
2. Description of Related Art
Protective circuits for electrical connection to solar cells of a solar cell module are well known from practice in diverse versions. These circuits are also partially called terminal circuits for solar cells.
Generally, several solar cells are interconnected into solar cell modules. In this regard, there is either the possibility of series connection or the possibility of parallel connection of the solar cells in the solar cell module. By parallel connection of the solar cells, the individual currents of the solar cells are added to one another to form a total current. In this connection, the solar cells connected in parallel should have essentially the same physical properties so that, in practice, parallel connection of solar cells has not prevailed, especially also since the individual solar cells can already deliver a current of a few amperes and the output voltage of individual solar cells is typically too small to be able to operate electrical devices, such as household appliances.
Therefore, when solar cell modules are assembled from individual solar cells, they are often series-connected. However, a problem can arise when a solar cell module is partially shaded in operation, so that one solar cell or a majority of solar cells of the solar cell module acquires less solar radiation or none at all, thereby affecting power output from the module. The reason for the solar irradiation being reduced in this way can be fouling of the solar cells and/or casting of shadows by trees, building facilities or building themselves.
In contrast to shading of the solar cell module uniformly over the entire area, which leads only to a power reduction overall, the following problem arises with partial shading: A common current flows through the series-connected solar cells of the solar cell module, each individual solar cell with its respective voltage contributing to the overall voltage of the solar cell module. At this point, if a solar cell is shaded, it no longer produces voltage and essentially opposes a diode in the reverse direction to the current flow in the solar cell module. This means that the entire solar cell module can no longer deliver current so that the entire operation of the solar cell module is adversely affected.
Furthermore, there is a voltage on the shaded solar cell which depends on the position of the shaded solar cell in the series circuit. If the voltage which is present on the shaded solar cell is greater than its reverse voltage, a breakdown, and thus, permanent damage will occur in the solar cell.
Even if the solar cell should not be damaged by a breakdown, in a shaded solar cell, a large power loss is converted so that the shaded solar cell heats up. This heating can also lead to damaging of the shaded solar cell and the solar cells adjacent to it.
In order to avoid the problems associated with a partially shaded solar cell, protective means are used, in general, specifically, bypass diodes which are connected anti-parallel to the solar cells. This results in that a shaded solar cell no longer contributes to the total solar cell voltage, but the current flow is nevertheless maintained. The solar cell module shows only a reduced operating voltage, but does not completely fail. Moreover, in the shaded solar cell, power is no longer converted so that damage to the shaded solar cell can be avoided.
Fundamentally, each solar cell of a solar cell module could be assigned exactly one bypass diode. However, often the procedure is such that a plurality of series-connected solar cells, i.e., a so-called string of solar cells, is safeguarded by a common bypass diode.
Protective circuits for electrical connection to solar cells of a solar cell module, generally, have at least one bypass diode, often a plurality of bypass diodes. However, this is associated with the problem that the protective circuits used for electrical connection of solar cells of a solar cell module are greatly heated by the power converted into the bypass diodes; this is disadvantageous in various respects.